The products of general formula (I) have been described as intermediates for the preparation of soluble derivatives of pristinamycin I.sup.A or of virginiamycin in European Patent Nos. 133,097 and 248,703.
According to the teaching of European Patient No. 133,097, the synergistic derivatives of general formula (I) may be obtained from natural pristinamycin I.sub.A (Y=dimethylamino) or from virginiamycin (Y=hydrogen) of general formula (II) mentioned below, via an enamine which is treated with an alkali metal borohydride in the presence of a strong organic acid.